Ketogenic Diet 101: A Complete Scientific Guide to Keto

Medically Reviewed

One enduring buzzword to hit the diet world seems to be “keto” — referring to the high-fat, low-carb ketogenic diet. With claims that you can eat all the fat you want, never feel hungry again, and even boost your athletic performance, the diet promises something for everyone.

But what exactly is the ketogenic diet, and is the weight loss program right for you? Let’s take a closer look before you attempt to make over your eating habits and lifestyle.

What Is the Ketogenic Diet?

The ketogenic diet is based on the principle that by depleting the body of carbohydrates, which are its primary source of energy, you can force the body to burn fat for fuel, thereby maximizing weight loss. When you consume foods that contain carbohydrates, the body converts those carbohydrates into glucose, or blood sugar, which it then uses for energy.

Because glucose is the simplest form of energy for the body to use, it’s always used for energy before your body turns to stored fat for fuel.

On a ketogenic diet, the goal is to restrict carbohydrate intake so that the body must break down fat for energy. When this occurs, fat is broken down in the liver, producing ketones, which are by-products of your metabolism. These ketones are then used to fuel the body in the absence of glucose.

What Is the Keto Diet?

What Is the Keto Diet?

How to Follow the Ketogenic Diet

There are several types of keto, but essentially, to achieve a state of ketosis, you have to severely reduce the amount of carbs you eat. (You can use a ketogenic calculator to create a custom food plan.) Data suggest the average American man age 20 or older consumes 46.4 percent of his daily calories from carbs, and the average American woman older than 20 consumes 48.2 percent of her daily calories from carbs.

But in the classic ketogenic diet, which was originally used to manage seizure disorders, 80 to 90 percent of calories come from fat, 5 to 15 percent come from protein, and 5 to 10 percent come from carbohydrates.

A modified version of the ketogenic diet, which allows you to eat protein more liberally — at 20 to 30 percent of your total calories — with the same carbohydrate restriction, is the more commonly used version of the diet today. Some of the aims of the latest version of the ketogenic diet are weight loss, weight management, and improved athletic performance.

What Is Ketosis?

The ketogenic diet for weight loss is based on the idea that driving the body into ketosis will maximize fat loss. Ketosis is a normal metabolic process that occurs when the body does not have enough glucose stores for energy. When these stores are depleted, the body resorts to burning stored fat for energy instead of carbs. This process produces acids called ketones, which build up in the body and can be used for energy.

How Do You Know if You’re in Ketosis?

To figure out whether you’re in a state of ketosis, check your urine for ketones. You can purchase ketone strips online or from a retail pharmacy. A strip that tests positive for ketones will indicate you have reached a state of ketosis.

Many people associate elevated ketones with a diabetic medical emergency known as ketoacidosis, but nutritional ketosis associated with a ketogenic diet and diabetic ketoacidosis are very different conditions.

Ketosis vs. Diabetic Ketoacidosis (DKA)

For people with diabetes, rapidly rising ketone levels can signal a health crisis that requires immediate medical attention. When there is an absence or not enough of the hormone insulin (or the body is too resistant to insulin to allow it to drive glucose into the cells for energy), the body cannot use glucose for fuel. Insulin helps ferry glucose to our cells and muscles for energy. Instead, in this case, the body resorts to burning stored fat for energy through the process of ketosis, leading to a buildup of ketones in the body.

As ketones accumulate in the bloodstream of a person with diabetes, they cause the blood to become more acidic, which can lead to the condition known as ketoacidosis. This condition can be fatal and should be treated immediately, as an article published in May 2021 in Endotext points out.

Learn More About Ketosis and How the Keto Diet Works

Potential Health Benefits and Risks of the Keto Diet

If you search online for the term “keto diet,” you'll find a lot of health claims associated with the ketogenic diet. But before you give this approach a try, it’s important to know what the science suggests about how it may affect your health. Namely, you'll want to know about potential keto diet dangers.

Risk: You May Suffer Fatigue and Other Symptoms as a Result of the Keto Flu

One of the most common side effects of starting the ketogenic diet is “keto flu.” This term describes the often unpleasant, fatigue-inducing symptoms that occur as the body adjusts from a high-carbohydrate to a low-carbohydrate diet. During keto flu, the body’s stored glucose begins depleting, and the body starts adapting to producing and utilizing ketones as energy.

Symptoms of the keto flu include headache, fatigue, dizziness, sleep problems, heart palpitations, cramps, and diarrhea. These side effects usually diminish and resolve in about two weeks.

But to lessen the effects of any discomfort, simply consider slowly transitioning onto a ketogenic diet rather than rushing to change your eating habits. By gradually lowering your carbohydrate intake and gradually increasing your intake of dietary fat, you can transition with less negative impact and potentially prevent the keto flu altogether.

Risk: You May Experience Constipation if You Don’t Eat Enough Fruits and Veggies

The removal of many grains and fruits with such a large emphasis on fats can bring about its own set of gastrointestinal side effects. Keto constipation and diarrhea aren't uncommon. “If not done properly — with most of your carbohydrates coming from fiber-rich vegetables — you may not be getting enough fiber, which can lead to constipation,” says Chris Mohr, RD, PhD, a sports dietitian based in Louisville, Kentucky, and the co-owner of

Risk: You Could Develop Dangerous Nutrient Deficiencies

Eliminating food groups can be problematic. “Ketogenic diets are often low in calcium, vitamin D, magnesium, and folic acid, which over time can lead to nutrient deficiencies if the diet is not planned carefully,” adds Marie Spano, RD, a sports performance nutritionist in Atlanta.

RELATED: What Is an Elimination or Exclusion Diet?

Risk: You May Harm Your Heart With the Diet’s Emphasis on Animal Fat and Protein

Reliance on a diet rich in animal fats and proteins may also have a negative impact on heart health, research shows.

“This diet is not for anyone who is at risk of developing cardiovascular disease or who has already been diagnosed with it,” Spano cautions.
This means that if you have risk factors for heart disease — such as elevated cholesterolhigh blood pressure (hypertension), or a strong family history of the disease — you should use caution when following this diet. The diet's heavy reliance on fat, especially saturated fat, can raise cholesterol levels, further increasing your chances of developing heart disease in the future.

A 2019 position statement from the National Lipid Association Nutrition and Lifestyle Task Force noted that research has found low-carb and very low carb plans (which keto fits into) have mixed effects on blood cholesterol levels, with some studies finding that these diets raise cholesterol.

RELATED: Is the Paleo Diet Good for Heart Health?

Risk: You May Experience Dangerous Low Blood Sugar if You Have Diabetes

For any individual with diabetes, discussing dietary changes — especially those as dramatic as the ones the ketogenic diet requires — with your healthcare team is essential. Because carbohydrates are broken down into glucose in the blood, cutting carbohydrates from your diet could cause levels to crash rapidly depending on your current medication regimen. Such a change may require significant adjustments to medication and insulin to prevent dangerous side effects such as low blood sugar, called hypoglycemia.

Risk: You May Experience Weight Cycling and Negative Effects on Your Metabolism

Outside of physical health changes, one of the biggest concerns of the ketogenic diet may be in long-term adherence. “It’s a very difficult diet to stick to and maintain. Compliance is a challenge because it is so restrictive,” explains Dr. Mohr.

Following a strict diet for weight loss and then quickly reverting to old habits when the dietary changes are too restrictive can lead to what is known as weight cycling, or yo-yo dieting. Gaining and losing the same weight over and over is associated with poorer cardiovascular health, especially among premenopausal women, according to a study in Preventive Medicine Reports published in December 2019.

Benefit: You May See Improvements in Your Athletic Performance 

For athletes, research on the keto diet highlights potential improvements in athletic performance, especially when it comes to endurance activities. An article in the British Journal of Sports Medicine found that ketogenic-type diets allowed endurance athletes to rely mostly on stored fat for energy rather than having to refuel with simple carbohydrates during endurance training and competition, and saw improved recovery times.

That said, a review published in October 2020 in Sports concluded that while the keto diet may help athletes reduce their weight and body fat, there is no conclusive evidence that the method of eating improves or harms health and performance.

Benefit: You Could Lose Weight Fast — but Not Necessarily More Than You’d Lose on Other Diets

If you’re looking to lose weight, one benefit the ketogenic diet may offer is appetite suppression. A review of this form of eating suggests it may help decrease appetite, but how this actually happens needs to be studied further.

Very low calorie ketogenic diets may help people who are overweight or have obesity reduce their BMI, decrease the circumference of their waist, and lower levels of A1C, total cholesterol, triglycerides, and blood pressure, notes a review and meta-analysis published in March 2020 in Reviews in Endocrine and Metabolic Disorders.

But when it comes to weight loss — one of the biggest keto selling points for many individuals — the benefits of the ketogenic diet may not be much different from any other diet plan. “There is no magical weight loss benefit that can be achieved from this diet,” says Spano. “The ketogenic diet may help weight loss in the same way other diets help — by restricting food choices so you eat fewer calories.”

Mohr agrees. “Cutting so many carbohydrates is a big reduction in calories,” he says, adding that this effect will lead to a loss of water weight up front, “which is why people like the immediate response of weight loss that comes from this type of diet.” That said, the calorie reduction isn’t always the case, as it can be easy to consume more calories overall if you’re eating more calorie-dense foods (namely fat). One small study published in February 2021 in Nature Medicine found that people ate almost 700 fewer calories per day on a plant-based low-fat diet compared with a low-carb one consisting of mostly animal fats.

Benefit: You May See Better Blood Glucose Control if You Have Type 2 Diabetes

For individuals with diabetes, adapting a very low carbohydrate diet, such as the ketogenic diet, may offer some benefits when it comes to glucose management. For instance, a previous review found that dietary restriction of carbohydrates may reduce or eliminate the need for medication in individuals with type 2 diabetes.

Learn More About the Possible Benefits and Risks of the Keto Diet

Common Questions & Answers

What can I eat on keto?
In short, a whole lot of fat, a moderate amount of protein, and very few carbs. Some of the best foods to eat on keto include avocado, olive oil, eggs, asparagus, green peppers, and cauliflower. On the flip side, you should avoid foods high in carbs — such as bread, pasta, many fruits, chips, crackers, candy, and cake.
Is it safe to do the keto diet?
For certain people, keto can pose health risks, so talk to your healthcare team first. Some groups that may want to avoid keto are: anyone with an underlying health condition like diabetes, pregnant or breastfeeding women, those who don't have a gallbladder, anyone with a history of an eating disorder, and anyone taking prescription medication.
Why is ketosis bad?
It's not bad exactly — unless you have kidney disease, as this metabolic state can strain the organs. But ketosis is essentially a natural metabolic state in which your body, in the absence of sufficient carbs, begins relying on fat for energy. It's different from diabetic ketoacidosis, which is a health emergency.
How do I get started on keto?
Your best chance for success on such a restrictive diet as keto is to find a registered dietitian familiar with the eating style. He or she will help you gradually change your diet so you're eating fewer carbs and more fat, staying hydrated, managing any keto side effects that come up, and meeting all your nutritional needs.
What are the side effects of a ketogenic diet?
An infamous stage of the keto diet is the keto flu. It's the collection of symptoms (think constipation, headaches, nausea) that you get when your body is adjusting to ketosis. Rest assured your symptoms will pass within a couple of weeks, but a registered dietitian can help you minimize them.

Is the Keto Diet Right for People With Diabetes?

Because the main tenet of the keto diet is counting and cutting carbs — a commonly used way to control blood sugar — this eating approach has become increasingly popular among people with type 2 diabetes who are looking to lower their A1C, which is the two- to three-month average measurement of blood sugar levels. Indeed, research suggests this diet may lead to fast weight loss and potentially lower blood sugar for people with the disease.

But dietitians warn the keto diet also comes with risks that are specific to people managing diabetes, including possible drug interactions and potentially dangerous low blood sugar if you're on medication, as well as kidney damage in people whose kidneys are dysfunctional because of elevated ketones in the blood.

Plus, because keto hasn't been studied long term, researchers don't know if the diet will result in nutrient deficiencies for those with or without diabetes.

If you're considering trying the keto diet and you have diabetes, it's crucial to consult your diabetes-care team before doing so to make sure it's a safe and effective eating approach for you.

Learn More About How the Keto Diet May Benefit People With Type 2 Diabetes

How to Get Started on the Ketogenic Diet

Here are some other things to know before you try this restrictive eating plan.

Can You Stick With the Carb Restrictions?

Following a diet that drastically restricts carbohydrates requires carefully monitoring your food choices to ensure you are meeting your nutritional needs. Work with a registered dietitian to make sure you follow this diet in a healthy manner without increasing your risk for complications or negative side effects. You can find a registered dietitian at

It’s important to remember that the goal of any dietary change is to promote a healthy lifestyle, so make sure to select a meal plan you can envision yourself following long term. If you know you will not be able to comply with such stringent carbohydrate restrictions for years to come, the ketogenic diet is most likely not the right choice for you.

What Are the Different Types of Keto Diets?

There are various modifications of the ketogenic diet. The majority of individuals following a ketogenic diet follow the so-called standard ketogenic diet plan, which provides about 10 percent of your total calories from carbohydrates.

Other forms of ketogenic diets include cyclic ketogenic diets, also known as carb cycling, and targeted ketogenic diets, which allow for adjustments to carbohydrate intake around exercise. These modifications are typically implemented by athletes looking to use the ketogenic diet to enhance performance and endurance and not by individuals specifically focused on weight loss.

Generally speaking, if you plan to follow a ketogenic diet, you should aim to consume less than 10 percent of your total calories from carbohydrates per day. The remaining calories should come from 20 to 30 percent protein and 60 to 80 percent fat. That means if you follow a daily 2,000-calorie diet, no more than 200 of your calories (or 50 grams) should come from carbs, while 400 to 600 calories should come from protein and 1,200 to 1,600 should come from fat. (There’s a reason this plan is also called a high-fat, low-carb diet!)

Is Exercise Involved in the Standard Ketogenic Diet?

Although the ketogenic diet does not explicitly require incorporating fitness into your routine, increasing your physical activity is always important when you want to reduce to or maintain a healthy body weight, according to an article in Progress in Cardiovascular Diseases.

For endurance athletes, the transition to a ketogenic diet may cut recovery time after training, but for casual exercisers, the transition to the ketogenic diet may make sticking with your fitness routine a challenge at first, the article notes. If you feel your energy levels drop too much when starting the ketogenic diet, slow down your reduction of carbohydrates, and make sure to do it gradually rather than all at once.

What Side Effects Should You Expect?

To prevent side effects such as the keto flu, begin transitioning your meal plan gradually. Start by understanding how many carbohydrates you consume most days. Then begin slowly reducing your carbohydrate intake over a period of a few weeks while gradually increasing your intake of dietary fat to keep your calories the same. You should also make sure to seek guidance from a professional to make sure this plan works for you and your health goals. “See a dietitian and adapt the diet to fit your long-term needs,” Spano recommends.

Learn More About What Beginners Should Know Before Trying the Keto Diet

What to Eat on the Standard Ketogenic Diet

The ketogenic diet is not a commercial meal plan, so there are no costs or membership fees associated with starting this diet. But, depending on your current eating habits, this eating approach may increase your food bill.

Because many processed foods are not considered ketogenic diet friendly, a switch to buying more whole, unprocessed foods may seem expensive, especially with the emphasis on high-fat and protein-rich foods.

In-season, fresh produce, along with frozen vegetables, which can be just as healthy as their fresh counterparts, will help reduce your costs. Although nuts, seeds, and animal proteins such as beef can drive up the grocery bill, bulk buying can help you save on these items as well.

The ketogenic diet relies heavily on dietary fat. Because high levels of animal fat in the diet have been associated with increased levels of cholesterol, aiming to include a good variety of plant-based fats can be helpful. Plant-based oils such as olive oil and avocado oil provide healthy fat for cooking and dressings.

Adding fat-rich foods such as avocado, nuts, and seeds can all make for healthful options that will provide you with unsaturated fats along with beneficial fiber. Most fruits are restricted on this plan — there are exceptions, including avocado — but nonstarchy vegetables such as leafy greens should become a staple of your diet.

Lean proteins such as fish, poultry, and grass-fed beef can be included as a source of protein on this diet.

A List of Acceptable Foods for the Standard Ketogenic Diet

  • Nonstarchy vegetables like leafy greens, broccoli, cauliflower, cabbage, peppers, mushrooms, onions, and rhubarb
  • Dairy, including eggs and cheese
  • Protein like beef, pork, poultry, fish, shellfish, and soybeans
  • Nuts and seeds, including walnuts, almonds, pistachios, sunflower seeds, and pumpkin seeds, coconut (in moderation)
  • Fats like plant-based oils and butter
  • Fruits like avocado, berries (in moderation), and tomatoes

Foods You Should Avoid or Limit on the Ketogenic Diet 

  • Processed foods like crackers, corn chips, and potato chips
  • Sweets, including candy, cookies, brownies, and cake
  • Grains of all kinds, including bread, pasta, rice, and quinoa
  • High-carb fruits like melons and tropical fruits
  • Artificial sweeteners such as Equal and Splenda

A Sample 3-Day Menu for the Standard Ketogenic Diet

Day 1

  • Breakfast: Scrambled eggs with sliced avocado
  • Snack: Almond butter on celery
  • Lunch: Spinach salad topped with canned tuna, olive oil, and vinegar
  • Snack: 1 ounce (oz) string cheese and 1 oz pistachios
  • Dinner: Sirloin steak paired with sautéed mushrooms, onions, and cauliflower rice

Day 2

  • Breakfast: Mushroom and cheese omelet with sliced bacon
  • Snack: ½ avocado
  • Lunch: Chicken stir-fry with peppers, onions, and peanuts sautéed in peanut oil
  • Snack: 1 oz Brie cheese with 1 oz walnuts
  • Dinner: Salmon fillet with oven-roasted Brussels sprouts

Day 3

  • Breakfast: Keto smoothie made with avocado, full-fat coconut milk, chia seeds, and nut butter
  • Snack: Hard-boiled egg
  • Lunch: Cheeseburger (without bun) over a bed of lettuce paired with string beans
  • Snack: 1 oz almonds
  • Dinner: Chicken breast paired with sautéed broccoli

Learn More About What You Can and Can’t Eat on the Keto Diet

What to Expect if You Try the Keto Diet

While the keto diet can lead to rapid weight loss through ketosis, the plan carries some health risks, including:

Because of the health risks involved, experts advise some individuals, such as those with heart disease or individuals who are at a higher risk for it, against trying the keto diet. People with type 2 diabetes should consult their doctor before attempting the keto (or any new) diet.

Because of the severe carb restrictions and elimination of food groups such as grains, the plan may also be challenging to adhere to in the long run. Trying the diet, giving it up, then trying it again may lead to weight cycling, or yo-yo dieting, which will make it harder to lose weight permanently.

If you are planning to try the keto diet, be sure to consult your healthcare team and, if possible, a registered dietitian to make sure you meet your nutritional needs on the plan. Working with a professional can help you determine whether you should make adjustments or if you’d be better off avoiding the diet entirely.

Learn More About What to Expect on the Keto Diet

Editorial Sources and Fact-Checking

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  • Low-Carb Diet: Can It Help You Lose Weight? Mayo Clinic. November 18, 2020.
  • Dietary Intake for Adults Aged 20 and Over. National Center for Health Statistics: Centers for Disease Control and Prevention. March 26, 2021.
  • Gosmanov A, Gosmanova E, Kitabchi A. Hyperglycemic Crises: Diabetic Ketoacidosis (DKA), and Hyperglycemic Hyperosmolar State (HHS). Endotext. May 19, 2015.
  • Constipation: Overview. Mayo Clinic. June 29, 2019.
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  • Heart Disease Risk Factors. Centers for Disease Control and Prevention. December 9, 2019.
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  • Paoli A. Ketogenic Diet for Obesity: Friend or Foe? International Journal of Environmental Research and Public Health. February 2014.
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